A team of US-based scientists have described the usefulness of human landing capture as an alternative method to collect mosquitoes that are potent vectors of West Nile virus. The study is currently available on the Research Square* pre-release server under review for publication in BMC parasites and vectors.

Study: Human biting mosquitoes and implications for WNV transmission. Image Credit: mycteria/Shutterstock

Background

West Nile virus is a mosquito-borne zoonotic virus that causes human and animal disease. Birds are the natural host of West Nile virus, and maintaining the viral life cycle in nature involves transmission between birds and mosquitoes. Mosquitoes belonging to Culex gender are the main vectors of this virus.

West Nile virus is highly endemic in North America. Since arriving in the United States in 1999, the virus has caused more than 52,000 infections in humans, with a fatality rate of 4.7%. Currently, the virus is circulating across the United States, including places monitored by strict mosquito control programs.

In the United States, surveillance for West Nile virus is done primarily by gravid traps and CO2– baited light traps. While gravid traps specifically collect Culex mosquitoes, light traps are designed for more generalized mosquito collection. Both methods collect many mosquitoes which are often damaged during the capture process. This makes the morphological identification of West Nile virus vector mosquitoes difficult.

In the present study, scientists evaluated the effectiveness of human landed catch collection methods in attracting and capturing Culex mosquitoes.

Human landing capture is a simple and accurate method to monitor and collect mosquito vectors. The method is performed by mosquito catchers who expose their limbs to attract and collect mosquitoes that come to feed on human blood.

study design

Human landing capture methods were performed between July and September 2018 and 2019. A total of 55 natural sites in the northwest suburbs of Chicago were selected for mosquito collection. All locations were in the Northwest Mosquito Control District, which was formed to control disease-causing mosquitoes.

The human collectors exposed one of the limbs weekly for 15 minutes at twilight periods (5:00 a.m. to 8:30 a.m. and 6:00 p.m. to 9:30 p.m.). Culex mosquitoes are most active during these times. They used a mechanical aspirator to collect mosquitoes within 3-4 seconds of landing on exposed skin.

The mosquitoes collected were subjected to morphological and genomic tests to distinctly identify Culex species. Most mosquitoes collected were notCulex species and were considered a nuisance to the public. The “nuisance factor” and “additional human West Nile virus risk” were calculated using mosquito bite rates and human host availability.

Data obtained from human landing capture methods were compared to historical data from mosquito collections via gravid traps and light traps from four locations in Chicago.

Important observations

A total of 223 mosquitoes were collected by human landing capture methods; 46 were Culex species. The majority of collected Culex the species were Culex saltwort (74%), generally considered less abundant in the upper US Midwest.

Of 55 locations selected, 33 were without Culex mosquitoes. During the entire mosquito collection period, a total of 2,821 individuals were present in the localities. Based on these estimates, the estimated range of the harm factor was 32 to 0. Similarly, the estimated range of the additional risk factor for human West Nile virus was 1.4 to 0.

Historical data used in the study included a total of 1,930,680 mosquitoes collected via gravid traps and light traps. About 76% of them were Culex mosquitoes. Gravid traps collected a greater number of Culex mosquitoes than light traps.

Considering each trapping effort, gravid traps and light traps collected a 7.5 times higher number of Culex mosquitoes than human landing capture methods.

Significance of the study

The study highlights that the human landing capture method effectively identifies and collects Culex mosquitoes, which are potent vectors of West Nile virus. Genetic and morphological analyzes of the mosquitoes collected identify Culex saltwort as a potential carrier of West Nile virus. The abundance of Culex saltwort mosquitoes has been underestimated in the United States because commonly used trapping methods are less effective at catching this particular species.

Given the potential health risks that can arise from mosquito-borne pathogens, scientists recommend implementing appropriate personal safety measures for human collectors before any routine application of human capture methods.

*Important Notice

Research Square publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be considered conclusive, guide clinical practice/health-related behaviors, or treated as established information.

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